We developed "Fracture-permeation" to investigate the intermolecular spaces of the cytoplasmic matrix. "Fracture-permeation" is the reversal of gel filtration: a macromolecule of known size and shape is used to assess the "mesh" of a chemically fixed cytoplasmic matrix. Cells and tissues are fixed in glutaraldehyde, impregnated in glycerol and frozen. The frozen speciment is freezefractured, thawed, and immersed in a concentrated solution of globular proteins. We use native ferritin (NF) and cationized ferritin (CF) to permeate model matrices of BSA, and to determine the compactness of the cytoplasmic matrix of (a) human lymphocytes and neutrophils; (b) isolated human lymphocytes activated by phytohaemagglutinin; (c) differentiation stages of the fungus Phytophthora plamivora and (d) striated muscle cells. With BSA matrices, we show that permeation depends on the diameter of the probe and on the intermolecular distances within the cross-linked matrix. Ferritin did not permeate the cytoplasm of mature human neutrophils, fugal zoospores and cysts, examples of cells with low levels of protein synthesis. In spores and cysts, examples of cells with low levels of protein synthesis. In resting lymphocytes, permeation of ferritin was limited or absent; it became massive in cells activated by phytohaemagglutinin. Massive permeation of ferritin was observed within the cytoplasm of other active cells (sarcoplasm of striated muscle, fungal sporangia, germinating cysts). Therefore, compactness of the cytoplasmic matrix varies through differentiation and is related to the degree of cellular activity. In striated muscle, Fracture-permeation revealed distinct patterns of intermolecular spaces for each state of contraction. The patterns observed for sarcomerses in rigor, stretched or at relaxed states can be inferred from the "sliding filament model" (Huxley, 1969). In cardiac muscle, the contracted state (with penetration of the A band) suggests unexpected irregularities of molecular interactions during contraction. Our results validate "Fracture-permeation" as a new method to investigate intermolecular distances in the cytoplasmic matrix.